Various circuits to control the operating point of color television circuits are known. Usually, to control the operating point of an output stage which, in turn, provides control current to the cathode of a television tube, a voltage representative of beam current is obtained and compared with a comparison reference. The result of the comparison is stored during the subsequent field of half-frame of a video image and the operating point of the video output amplifier is controlled as a function of the stored value. The test of the beam current is carried out during scanning of a line within the vertical scanning interval, just after the vertical retrace of flyback of the beam.
Image viewing-type television apparatus, for example for home use or the like, usually use a scanning pattern which is somewhat larger than the area visible to the viewer, that is, the area which is left blank from between the frame. Thus, the scanned pattern is written on the screen in a region which is larger than that which is actually visible. It is, thus, readily possible to test the beam current at the beginning of the first line, after vertical flyback or retrace, in order to obtain a suitable measuring value for comparison with the reference, which may cause distortion on the screen. This distortion, however, is not visible to the viewer because the first line is, at least in part, blanked by the frame of the television apparatus.
Data output apparatus display the entire scanned pattern on the screen. Thus, testing and scanning for the beam current in order to obtain a measuring line, so that a voltage value representative of beam current can be obtained, cannot be carried out at the edges of the scanned pattern without also rendering visible that particular line.
Data monitors and data display apparatus, thus, in order to prevent interference with a scanning line which is being displayed, frequently did not utilize known TV-type control apparatus to control the operating point of video output stages.